Sunstruck Flavor of Rice Vinegar

It was found that the precursor of the sunstruck flavor was formed in the course of maturing sake cake. A large quantity of yeasts in the sake cake was autolysed and VB₂ was evolved. Like the sunstruck flavor of milk reported by Patton et al. it is thought that, VB₂, a photochemical sensitization substance caused methionine to react photochemically and to evolve the sunstruck flavor 3-(methylthio)-propanal (methional). Organoleptically, vinegar with added methional an vinegar which had evolved the sunstruck flavor had the same odor.     

Contribution to Stale Flavor of 2-Furfuryl Ethyl Ether and Its Formation Mechanism in Beer

The effect of storing beer on the generation of stale flavor was investigated by a sensory evaluation and quantitative determination of the volatile components. The results of the sensory evaluation revealed 2-furfuryl ethyl ether (2-FEE) to be an important component responsible for the astringent stale flavor, and (E)-2-nonenal (E2N) for the cardboard-like stable flavor. However, the stale flavor could not be reproduced by adding 2-FEE or E2N alone to fresh beer, but was closely reproduced when 2-FEE and E2N were added together. The results of experiments on adding furfuryl acetate (FAC) and furfuryl alcohol (FAL) to a 5% ethanol solution or beer indicate that 2-FEE was generated by the reaction between ethanol and FAL, which was formed by the hydrolysis of FAC, during the storage of beer.     

低乙醛Lager型啤酒酵母研究进展

啤酒酵母是啤酒酿造的核心,对啤酒风味及风味稳定性具有重要影响。乙醛是影响啤酒风味和风味稳定性最重要的醛类化合物,是酒精饮料中引起人类致癌的物质之一,主要通过啤酒酵母的生物代谢产生,存在于啤酒发酵过程及成品啤酒中。因此,筛选或选育优良的低产乙醛啤酒酵母菌株将成为有效解决啤酒风味稳定性的途径之一。近年来,随着基因工程技术的发展及啤酒酵母基因组的不断阐明,人们对啤酒酵母菌种改良展开了大量的研究,以期解决啤酒酿造问题,改善啤酒质量。本文对采用传统方式及基因工程手段选育低产乙醛啤酒酵母的最新研究进展进行了综述。其中,对低乙醛啤酒酵母选育的手段及策略进行了讨论并对低乙醛啤酒酵母选育的研究热点及发展趋势进行了展望。     

采用代谢组学分析技术分析工业啤酒发酵过程中风味物质生成规律

啤酒风味是保证啤酒品质的关键因素之一。运用代谢组学的方法,分析工业啤酒发酵过程中酵母胞内代谢物和啤酒风味物质的对应关系,从代谢水平上研究风味物质形成过程中的关键影响因素。在啤酒发酵过程中,同时检测风味物质的含量变化和酵母胞内代谢物的变化,对得到海量的、多维的代谢数据采用主成分分析（PCA）和偏最小二乘分析（PLS）的多元统计分析方法进行处理。由PCA分析结果可知：磷酸、海藻糖、琥珀酸、谷氨酸、天冬氨酸、丙氨酸对主成分贡献比较大,说明这些代谢物在不同发酵阶段含量变化显著。由PLS分析结果可知：对啤酒风味影响最大的物质主要为氨基酸,包括丝氨酸、缬氨酸、苏氨酸、赖氨酸、丙氨酸、亮氨酸和天冬酰胺等,这为啤酒中风味物质的调控提供了一定的理论指导。     

Reduced urination rate while drinking beer with an unpleasant taste and off-flavor

A lowered subjective evaluation of the taste and flavor of beer due to staleness or to the addition of an unpleasant taste and flavor was found to be closely correlated with the urination rate. Beer in the same lot was compared immediately after shipment from the brewery and after leaving at room temperature for 1 month or 5 months. Each beer sample was given to volunteers at the rate of 3 ml/kg/15 min for 2 hours, and the urine volume was measured every 30 minutes. The urination rate was highest from the volunteers who drank fresh beer and lowest from those who drank 5-month-old beer. The subjective evaluation of both the taste and drinkability of 5-month-old beer was significantly lower than that of fresh beer. Beer samples with various unpleasant taste and flavor substances added lowered the urination rate. The results suggest that the perception of an unpleasant taste and off-flavor would lower the urination rate.     

Absence of fks1p in lager brewing yeast results in aberrant cell wall composition and improved beer flavor stability

The flavor stability during storage is very important to the freshness and shelf life of beer. However, beer fermented with a yeast strain which is prone to autolyze will significantly affect the flavor of product. In this study, the gene encoding β-1,3-glucan synthetase catalytic subunit (fks1) of the lager yeast was destroyed via self-clone strategy. β-1,3-glucan is the principle cell wall component, so fks1 disruption caused a decrease in β-1,3-glucan level and increase in chitin level in cell wall, resulting in the increased cell wall thickness. Comparing with wild-type strain, the mutant strain had 39.9 and 63.41 % less leakage of octanoic acid and decanoic acid which would significantly affect the flavor of beer during storage. Moreover, the results of European Brewery Convention tube fermentation test showed that the genetic manipulation to the industrial brewing yeast helped with the anti-staling ability, rather than affecting the fermentation ability. The thiobarbituric acid value reduced by 65.59 %, and the resistant staling value increased by 26.56 %. Moreover, the anti-staling index of the beer fermented with mutant strain increased by 2.64-fold than that from wild-type strain respectively. China has the most production and consumption of beer around the world, so the quality of beer has a significant impact on Chinese beer industry. The result of this study could help with the improvement of the quality of beer in China as well as around the world.     

ILV2基因缺失对啤酒酵母生长与双乙酰代谢的影响

【目的】旨在应用分子生物学方法降低啤酒发酵液中双乙酰含量,改善啤酒感官质量。【方法】以酿酒酵母S2(Saccharomyces cerevisiae)为出发菌株,通过同源重组敲除四倍体啤酒酵母α-乙酰乳酸合成酶部分基因(ILV2),构建缺失一个和两个ILV2等位基因的突变株QI2-1和QI2-2,并进行啤酒发酵实验。【结果】ILV2基因的缺失,会导致菌株初始生长速率的降低。其中QI2-2较为明显,12 h时,突变株与出发菌株的生长速率达到一致。啤酒发酵结果表明,与出发菌株相比,突变株QI2-1双乙酰峰值与双乙酰最终含量分别降低17.50%和17.83%,而QI2-2分别降低51.67%和45.65%。其他啤酒指标如酒精度、发酵度、残糖和风味物质等略有变化,但都在优质啤酒指标范围内,符合啤酒发酵的质量要求。【结论】通过同源重组敲除部分ILV2基因和选育低产双乙酰菌株是降低啤酒双乙酰含量、提高啤酒质量的有效方法,具有一定的实际应用价值。     

几株啤酒酵母的筛选及发酵性能比较*

啤酒酵母是啤酒酿造的灵魂,可以直接影响啤酒品质。在啤酒酿造过程中,由于啤酒酵母被多次传代和保藏,造成优良菌种发酵性能衰退等问题,导致发酵不彻底,影响最后啤酒的风味质量。为此以8株Lager型啤酒酵母为出发菌株,通过平板分离纯化获得80株分离菌株,再经过三角瓶发酵初筛和复筛、发酵罐中试发酵实验最终获得了8株发酵性能优良的啤酒酵母。其中,6株酵母可应用于酿造双乙酰含量低于0.1 mg/L的啤酒;3株酵母发酵度高于70%,适合酿造干啤酒;1株酵母发酵度低于50%,适合酿造低醇啤酒。在风味方面:1株酵母酿造的啤酒醇酯比为3.3,啤酒酯香味较突出;另1株酵母酿造的啤酒醇酯比为4.5,啤酒高级醇含量较高。8株经过选育的啤酒酵母发酵特征明显,便于精酿啤酒厂实际应用。     

多元分析用于啤酒风味的研究

通过检测国内外30种不同品牌啤酒中13种主要风味物质,采用多元统计方法中的主成分分析,得到3个主成分及综合风味特征得分;对这些酒样进行感官品评并采用频数分析计算得分．结果发现两种评分结果具有较好的对应性,初步表明根据啤酒中13种风味物质的检测结果,基本能够对啤酒的整体风味进行评判与质量监控．     

Characterization of 9-fatty acid hydroperoxide lyase-like activity in germinating barley seeds that transforms 9(S)-hydroperoxy-10(E),12(Z)-octadecadienoic acid into 2(E)-nonenal

Previously, we reported that 2(E)-nonenal, having a low flavor threshold (0.1 ppb) and known as the major contributor to a cardboard flavor (stale flavor) in stored beer, is produced by lipoxygenase-1 and a newly found factor named 9-fatty acid hydroperoxide lyase-like (9-HPL-like) activity in malt. To assess the involvement of 9-HPL-like activity in beer staling, we compared the values of the wort nonenal potential, an index for predicting the staleness of beer, with the lipoxygenase and 9-HPL-like activity of 20 commercial malts. There was a significant correlation between the malt 9-HPL-like activity and the values of wort nonenal potential (r=0.53, P<0.05), while the correlation between malt lipoxygenase activity and the wort nonenal potential was statistically insignificant. Analysis of the partially purified 9-HPL-like activity from embryos of germinating barley seeds indicated that 9-HPL-like activity consisted of fatty acid hydroperoxide lyase and 3Z:2E isomerase.     

Cytosolic localization of acetohydroxyacid synthase Ilv2 and its impact on diacetyl formation during beer fermentation

Diacetyl (2,3-butanedione) imparts an unpleasant "butterscotch-like" flavor to alcoholic beverages such as beer, and therefore its concentration needs to be reduced below the sensory threshold before packaging. We examined the mechanisms that lead to highly elevated diacetyl formation in petite mutants of Saccharomyces cerevisiae during beer fermentations. We present evidence that elevated diacetyl formation is tightly connected to the mitochondrial import of acetohydroxyacid synthase (Ilv2), the key enzyme in the production of diacetyl. Our data suggest that accumulation of the matrix-targeted Ilv2 preprotein in the cytosol is responsible for the observed high diacetyl levels. We could show that the Ilv2 preprotein accumulates in the cytosol of petite yeasts. Furthermore, expression of an Ilv2 variant that lacks the N-terminal mitochondrial targeting sequence and thus cannot be imported into mitochondria led to highly elevated diacetyl levels comparable to a petite strain. We further show that expression of a mutant allele of the γ-subunit of the F(1)-ATPase (ATP3-5) could be an attractive way to reduce diacetyl formation by petite strains.     

啤酒腐败微生物与啤酒微生物稳定性研究进展

尽管生产环境和卫生条件已经得到大幅改善,但啤酒生产过程中仍然会发生微生物污染,因此,真正意义上的啤酒纯种酿制是很难实现的。为了有效控制生产过程中的微生物污染,本文系统介绍了啤酒微生物的多样性及其在生产工序中的分布,探讨了啤酒环境对抑制啤酒微生物污染的影响,讨论了啤酒微生物对啤酒质量与风味的积极贡献,提出合理控制外源微生物侵染是形成不同啤酒典型特征的关键。     

Utilization of brewer's yeast cells for the production of food-grade yeast extract. Part 1: Effects of different enzymatic treatments on solid and protein recovery and flavor characteristics

Yeast extract was produced from brewer's yeast of a beer factory by combined enzymatic treatments using endoprotease, exoprotease, 5'-phosphodiesterase, and adenosine monophosphate (AMP)-deaminase. Effects of enzyme combination, enzyme dosages and treatment sequence on the recovery of solid and protein, flavor and compositional characteristics were investigated. Exoprotease dosage strongly affected the recovery of protein and degree of hydrolysis (DH) and sensory characteristics. When the yeast cells were treated using optimal combination of endoprotease and exoprotease (0.6% Protamex and 0.6% Flavourzyme), high solid recovery (48.3-53.1%) and the best flavor profile were obtained. Among various treatment sequences using multiple enzymes, treatment with protease followed by nuclease resulted in the highest 5'-guanosine monophosphate (5'-GMP) content. The optimal concentrations of both 5'-phosphodiesterase and AMP-deaminase were found to be 0.03%. After treatments using optimal combination of enzyme, enzyme dosages and treatment sequence for four enzymes, a high solid yield of 55.1% and 5'-nucleotides content of 3.67% were obtained.     

Construction of amylolytic industrial brewing yeast strain with high glutathione content for manufacturing beer with improved anti-staling capability and flavor

Glutathione in beer works as the main antioxidant compounds which correlates with beer flavor stability. High residual sugars in beer contribute to major non-volatile components which correlate to high caloric content. In this work, Saccharomyces cerevisiae GSH1 gene encoding glutamylcysteine synthetase and Scharomycopsis fibuligera ALP1 gene encoding alpha-amylase were co-expressed in industrial brewing yeast strain Y31 targeting at alpha-acetolactate synthase (AHAS) gene (ILV2) and alcohol dehydrogenase gene (ADH2), and new recombinant strain TY3 was constructed. The glutathione content from the fermentation broth of TY3 increased to 43.83 mg/l compared to 33.34 mg/l from Y31. The recombinant strain showed high alpha-amylase activity and utilized more than 46% of starch after 5 days growing on starch as sole carbon source. European Brewery Convention tube fermentation tests comparing the fermentation broth of TY3 and Y31 showed that the flavor stability index increased to 1.3 fold and residual sugar concentration were reduced by 76.8%, respectively. Due to the interruption of ILV2 gene and ADH2 gene, the amounts of off-flavor compounds diacetyl and acetaldehyde were reduced by 56.93% and 31.25%, comparing with the amounts of these from Y31 fermentation broth. In addition, as no drug-resistance genes were introduced to new recombinant strain, consequently, it should be more suitable for use in beer industry because of its better flavor stability and other beneficial characteristics.     

Bioflavoring and beer refermentation

Various techniques are used to adjust the flavors of foods and beverages to new market demands. Although synthetic flavoring chemicals are still widely used, flavors produced by biological methods (bioflavors) are now more and more requested by consumers, increasingly concerned with health and environmental problems caused by synthetic chemicals. Bioflavors can be extracted from plants or produced with plant cell cultures, microorganisms or isolated enzymes. This Mini-Review paper gives an overview of different systems for the microbial production of natural flavors, either de novo, or starting with selected flavor precursor molecules. Emphasis is put on the bioflavoring of beer and the possibilities offered by beer refermentation processes. The use of flavor precursors in combination with non-conventional or genetically modified yeasts for the production of new products is discussed.     

New Method for Analyzing the Volatiles in Beer

A new method is reported for analyzing the medium- and high- boiling-point volátiles in beer, using CCl₃F for extracting the whole volatiles and then the headspace trapping method with Tenax TA. This method seems to be effective for analyzing the flavor not only of beer but also of other beverages, especially those containing a large amount of ethanol.     

啤酒花抗性机制的研究进展

酒花苦味酸是构成啤酒风味的重要组分,也是啤酒生产过程中的天然抑菌剂,酒花苦味酸通过降低pH梯度而抑制啤酒花敏感菌生长。研究发现,啤酒花抗性菌是通过膜上转运蛋白将酒花苦味酸泵出细胞外,以降低膜上的质子流速,维持了细胞内的pH梯度。结合近年来酒花抗性相关研究结果,讨论了细胞膜上酒花抗性相关组分与酒花抗性间的关系,提出了酒花抗性机制的模型。     

啤酒酵母代谢工程研究进展

啤酒工业上应用的啤酒酵母菌株在生产中都会存在着某些方面的缺陷。通过分析啤酒酵母某些代谢产物的代谢途径,寻找改变其代谢流量的方法,然后用分子生物学手段对其代谢流量加以改变,来调节啤酒酵母某些产物的代谢水平已经成为啤酒酵母育种的新方式。对酵母的底物利用、可操作性、控制有害副产物的产量及改善啤酒风味等方面的研究成果进行了综述。     

Hop resistance and beer-spoilage features of foodborne <Emphasis Type="Italic">Bacillus cereus</Emphasis> newly isolated from filtration-sterilized draft beer

To meet consumer demands for fresher and healthier foods, the pasteurization of beer has largely been replaced by membrane filtration methods. Research on beer spoilage caused by hop-resistant bacteria is thus of great interest. In this context, we have isolated a beer-spoilage strain of bacteria from turbid bottled beer treated by membrane filter sterilization, which we identified as Bacillus cereus strain 3012. Strain 3012 was able to produce acids and bioamines, resulting in a change in the flavor profile of the beer. Unlike other known strains of B. cereus, this isolate was resistant to hop compounds present in the beer and was in an evolutionarily stable state in terms of hop resistance. Our results suggest that this hop-resistant B. cereus strain 3012 is a foodborne pathogen with the potential to cause beer-spoilage incidents in the brewing industry.